Use <time> element

Put post timestamp inside <time> element, instead of just
<span class="timestamp">.
Added header, footer, top, and bottom files

header.html is inserted in blog post pages' <header> element, after
the post title and timestamp, and footer.html is used as the content
of the post <footer> element.  top.html and bottom.html are inserted
before and after the <article> element.